Minimizing the temporal width of the gating window during calculation of the gating function in cardiac CT may not result in best image quality with a maximum of signal-to-noise ratio and minimal artifacts. According to an exemplary embodiment of the present invention, the widths of the gating windows in cardiac CT are selected on the basis of a motion model describing the motion of the heart. According to an aspect of the present invention, the widths of the gating windows are determined by considering the duration of stationary cardiac phases. Thus, artifacts, such as motion artifacts or noise, are minimized.